As described in Patent Document 1, in an internal combustion engine for an automobile engine, etc., the joint surfaces of flanges fastened by a bolt undergo a phenomenon to be a fretting state in which fretting caused by horizontal sliding (horizontal fretting) or opening (increase in the distance between the joint surfaces) occurs repeatedly because of vibration, thermal expansion, or the like. In order to prevent with this phenomenon, a metal gasket is used between the flange joint surfaces to seal fluid flowing in a fluid passage. In this case, however, there are problems that friction occurs repeatedly between the flange joint surfaces and a bead portion of the metal gasket, particularly in the projected portion of the bead portion, so that the rubber layer bonded to the metal plate of the metal gasket is worn and peeled off early in the projected portion, causing a significant reduction in sealing properties.
Patent Document 1 mentioned above proposes a composite material for a metal gasket with a bead, the composite material comprising a rubber layer and a solid lubricant layer bonded to the rubber layer, wherein the solid lubricant layer comprises PTFE (9 to 40 wt. %), carbon black (1 to 10 wt. %), a urethane resin (35 to 57 wt. %), a silicone resin (5 to 40 wt. %), and NBR (5 wt. % or less) or graphite (3 wt. % or less), and has a friction coefficient of 0.15 or less.
In addition to this proposal, the following various proposals are made to solve the problem of fretting wear (surface damage generated when a slight relative motion is periodically repeated between two contact surfaces).
(a) A method for reducing friction caused by fretting, by applying an inorganic powder-based solid lubricant, such as graphite or molybdenum disulfide, to the surface of the bonded rubber layer.
According to this method, the inorganic powder-based solid lubricant has a low friction-reducing effect, and the effect is not expected until familiarity (transfer) of the solid lubricant to the flange joint surfaces is generated. Accordingly, when high-load fretting occurs in the initial state, the bonded rubber layer is worn.
(b) A method for reducing friction caused by fretting, by applying a liquid lubricant, such as wax, oil, or fatty acid, to the surface of the bonded rubber layer.
According to this method, the liquid lubricant undergoes long-term deterioration when the metal gasket is used under severe temperature conditions, and the effect of reducing friction is lost. Depending on the type of fluid to be sealed, the liquid lubricant is eluted and lost into the fluid. Consequently, friction becomes high, resulting in the wear of the bonded rubber layer.
(c) A method for improving wear resistance by increasing the degree of vulcanization of the bonded rubber layer or raising the amounts of carbon black and other filler components in the rubber composition to thereby enhance hardness of the bonded rubber layer.
According to this method, the bonded rubber layer with a high hardness causes a decrease in sealing properties in low surface-pressure areas and in sealing properties in rough surfaces. As a result, the original sealing function of the metal gasket is impaired.
(d) Methods disclosed in Patent Documents 2 and 3 in which the front and back sides of a metal gasket have different frictional resistances so that the metal gasket can slide with respect to one of two joint surfaces that hold the metal gasket.
These methods have drawbacks. That is, since the wear of the bonded rubber layer caused by fretting is absorbed only by one side of the gasket, the degree of decrease in sealing properties become lower; however, this is not a fundamental solution because the wear of the rubber layer is not completely eliminated. Moreover, when the surface of the gasket on the side on which fretting occurs has a part for which high seal performance is required (a high surface pressure seal part, a fuel seal part, etc.), these methods cannot be employed.